Iso-octane (such as 2,2,4-trimethylpentane) is one of the most important components in gasoline and is the gasoline component for which the octane rating has been coined. While gasoline has many other components, fuels for piston engine airplanes can be pure iso-octane. The branches in iso-octane give it the appropriate combustion properties for ignition engines. Unfortunately, the branches in iso-octane also make it nearly impossible to produce biologically. Indeed, tertiary butyl carbons exist rarely in nature. The biological production of iso-octane and molecules like it would be extremely valuable to the transportation fuels industry as it would allow direct substitution of these biofuels into the existing transportation and refining infrastructure.